Laravel, to name one, broke HHVM compatibility a while ago (or perhaps more accurately no longer included it as a target platform). In my experience, trying it out every year or so with an existing project, there were always compatibility issues with HHVM, most often in C modules but also a few odd corner cases in pure PHP.

As far as I know once PHP 7 came out HHVM fell much further behind (as far as the PHP language goes, no idea how Hack is doing).